Chlorpyrifos and structurally related compounds have insecticidal activity against a variety of arthropods. Because of their extensive use in agriculture, it is important to be able to monitor their levels in water and food that could be ingested by humans. To that end, immunoassays utilizing antibodies against such phosphorothioate compounds as chlorpyrifos, chlorpyrifos-methyl, fenitrothion, and pirimiphos-methyl have been developed (J.H. Skerrit et al, Journal of AOAC International, Vol 75, 519 (1992); J. H. Skerrit et al; abstracts of The 106th AOAC Annual International Meeting & Exposition, Cincinnati, Ohio, 1992. p.162; PCT application PCT/AU90/00278 of J. Skerrit et al).
Normally, in an immunoassay, known quantities of the substance being assayed for; i.e., the analyte, are used as standards to calibrate the assay. Not all phosphorothioate insecticides are stable in aqueous conditions, however. Therefore, it is desirable to have a substance that will both react with antibodies such as anti-chlorpyrifos antibodies and be stable.